I've heard numerous folks who attended Build in person and/or via Webcasts say that there will be no Desktop app experience when Windows 8 ships on PCs and tablets running on ARM processors. Until today, I thought the same. But this is not correct.
Microsoft officials have been saying for months that existing x86 applications won't just run on Windows on ARM; they will need to be recompiled. Microsoft is hoping and expecting that the majority of devs will go to the trouble of "Metro-izing" their apps while they are recompiling them to run on Windows 8 on ARM. However, that is guidance, and not a requirement.
I saw a screen shot of the Win 8 ARM tablets being shown at Build that was sporting a Desktop tile over at ThisIsMyNext. It looks just like the Start screen on Windows 8 on the Intel i5-based Samsung tablets given to Build attendees (a screen shot of which is below, courtesy of my ZDNet colleague Ed Bott). Tapping the Desktop tile/accessing the Desktop "app" takes a user to the classic Windows mode.